Slow Internet Due to Local DNS Issue?

I have 2 Mbps Broadband Internet Connection. Recently , we have noticed our
Internet speed is slow. So we contacted our ISP. Our current connection is Modem
.... Firewall.. Switch.. Servers and clients.

ISP told us to check the line by connecting it to a single system . On that
system I am getting everything and I could download things speedily. So I
understood thats not ISP issue. So check with Firebox.. From one machine in
the LAN I removed local DNS servers IP and gave public DNS . On that
Machine I could access everything.If I give my Local DNS IP it will be a
slow Internet .

Dude! Are you nuts? Just how can you have high speed internet with a modem
connection? Not possible....Ahh! You mean cable modem. My bad. Is it slow for
everyone? Or just certain users? Check users to see that they are not
streaming music, video, etc. We have 2 T-1 lines yet internet gets extremely
slow when users are listening to Pandora music, or download material, or
streaming video. I have learned that if you leave equipment in their standard
settings, things will work by themselves. You might have a routing problem
here. Are all your NICS at the same speed? Gigabit, or fast ethernet? Ahh!
Maybe when when you use local DNS you are having the server go out and fetch
the data thereby causing your bottle neck. Understand that every computer has
to go to that local server to get the domain name names. The bottle neck is
when users are having to wait for one NIC to do all the stuff. If you use
public DNS, any computer can go and get the info without waiting for the
server. Tada! There you have it. Use Public DNS.

While you are troubleshooting this, don't forget infrastructure changes that may have injected interference into this. Has anyone worked on lighting in the vicinity of your cable runs? Have any pieces of equipment with a heavy electrical draw been added near any unshielded cable runs?

I am not suggesting the problem isn't about the 'network equipment' or settings, but a sudden change in network performance, shy equipment failure, can often be physical plant.

There can be connectivity issue, because you connect direct on single
computer its working fine, chekc all the connectivity...and if you have
realized that DNS cab be issue.you should check some website by IP.if
possible unpluged all the device form ur network and check one by one...

I too faced the same problem which was a nightmare for me. If you connect to one system we can't notice this issue. My advice is check what is the upload from your end with the ISP they will provide. If the upload is more that your usage then there must be some broadcast happening at your end.

If I give my Local DNS servers as PDNS and SDNS it will not work (means
Browsing breaks , Slow , No download). If I use my firewalls or ISP's public
IP as DNS it will work .What does this means please advise?

Hi Ambii,
Try this short experiment.
On a test system, disable IPv6 and change the TCP/IP DNS settings. Set the primary DNS to your local LAN DNS server. Set the secondary to your ISP's DNS server and add a third DNS such as google (8.8.8.8).
Try your connection again. Is it better?

Before SLA following should provide by ISP:
Flat Free - no download/upload volume limit.
No shared line, if possible (1:1)
All ports have been opened, no restrictions or access list.
ISP must provide 10/100 BaseT interface. Speed has to be fixed 100/ full
duplex.

Go and look at your individual ports on the switch and see if you have a large amount of traffic on a particular port. Also look at the ARP table and see if anything is out of the ordinary. You can also download namebench from google and run it to see what the optimal external DNS servers are. Often times the ISP will have you use theirs but if it is a smaller ISP they do not have the best DNS servers because they just forward the requests instead of actually setting up root server requests.

Have read all 26 responses...following could have happened:
1) Speed setting and duplex setting between isp and the modem
2) The dns server services may not be working properly...as it needs to forward request to external dns if out of domain lookip.

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